Stovetop Broccoli Casserole

Place broccoli and tap water in a large microwavable bowl. Cover loosely with plastic wrap, and poke holes in plastic wrap to vent (alternatively, partially cover with a lid). Microwave on HIGH until fork-tender, about 8 minutes. Uncover and set aside. 

While broccoli microwaves, melt 1 tablespoon of the butter in a medium-size Dutch oven or enameled cast-iron skillet over medium. Stir in crushed crackers; cook, stirring occasionally, until crackers are lightly toasted, about 2 minutes. Remove from heat; stir in chopped parsley. Transfer mixture to a small bowl, and set aside. Wipe skillet clean.

Add remaining 4 tablespoons butter to Dutch oven or skillet, and melt over medium-high. Add onion; cook, stirring occasionally, until softened, about 5 minutes. Add garlic; cook, stirring occasionally, until fragrant, about 1 minute. Sprinkle mixture evenly with flour; cook, stirring constantly, until mixture is light brown, about 1 minute. Gradually whisk in milk, and bring to a simmer over medium-high. Simmer, whisking constantly, until mixture is very thick, about 2 minutes. Reduce heat to medium-low; stir in Dijon mustard, salt, pepper, and 2 cups of the cheese. Cook, stirring constantly, until cheese is melted and smooth, about 2 minutes (mixture will be very thick). Stir in mayonnaise. Fold in steamed broccoli until well incorporated. Cook, stirring occasionally, until sauce has re-thickened to very thick, 12 to 15 minutes. Season with additional salt to taste.

Sprinkle casserole with remaining 1 cup cheese. Remove from heat, and immediately cover; let stand, covered, until cheese is fully melted, about 10 minutes. Uncover and sprinkle with seasoned crackers. Serve warm.